The most comprehensive marine survey ever done has revealed the incredible biodiversity of the sea around New Caledonia and shed light on the best conservation strategies for the area.

The survey involved between 35 and 45 researchers from nine different countries, led by mollusc expert Professor Phillipe Bouchet, of the Natural History Museum in Paris.

The researchers brushed rocks, did night dives, swept the sea grass, and trawled the soft sediments on the floor of a bay off the small island of Lifou.

"Basically we covered all bases," said one of the four Australians involved, Dr Thomas Schlacher of the Sunshine Coast University.

Ecologists equate the biodiversity of a region by the number of species found in it.

"But," said marine scientist Dr Schlacher, "the number of species you find depends on the sampling effort you put into it." He added that the number of samples identified depended on the taxonomic expertise available after collection.

Dr Schlacher described the project as a "benchmarking exercise" to see how many species could be found in one small area.

The survey collected 150,000 specimens of molluscs, which have since been classified into 3,000 different species, representing 50 per cent more than the species in the Mediterranean, and about five times the number in the whole of the British Isles.

The team were based in a local village during the five weeks it took to do the survey.

"There was a great mob of us there," said Dr Schlacher. "It was really rustic, fighting off the cockroaches. It was a rough few weeks."

Many of the species found are exceptionally rare. 30 per cent of the fauna was found at only one place in the bay. Not only are they restricted in their distribution but there are only a few specimens of each species. Of the 3000 species collected, 28 per cent were seen only once.

Dr Schlacher said this was the "worst case scenario" for conservation, which traditionally relied on keeping a representative range of habitats in the belief that this would conserve some of each species.

"But if you take a situation like this where there is basically a chequerboard of animals which have low abundances, by taking a subset of the area, you are not protecting the entire community," explained Dr Schlacher.

"We need to look at quite comprehensive areas."

Dr Schlacher has proposed a "chain of reserves" where all main islands have an area that is protected and each area is linked by channels, forming a chain that larva can flow along.

The "chain of reserves' would try to encompass different environmental conditions and its location would depend on the current, ocean depth and the age of the island.